Neurological stimulators have been developed to treat pain, movement disorders, functional disorders, spasticity, cancer, cardiac disorders, and various other medical conditions. Implantable neurological stimulation systems generally have an implantable signal generator (sometimes referred to as an “implantable pulse generator” or “IPG”) and one or more leads that deliver electrical pulses to neurological tissue or muscle tissue. For example, several neurological stimulation systems for spinal cord stimulation (SCS) have cylindrical leads that include a lead body with a circular cross-sectional shape and one or more conductive rings spaced apart from each other at the distal end of the lead body. The conductive rings operate as individual electrodes and, in many cases, the SCS leads are implanted percutaneously through a large needle inserted into the epidural space, with or without the assistance of a stylet.
Once implanted, the signal generator applies electrical signals to the electrodes, which in turn modify the function of the patient's nervous system, such as by altering the patient's responsiveness to sensory stimuli and/or altering the patient's motor-circuit output. In pain treatment, the pulse generator applies electrical signals to the electrodes, which in turn can generate sensations that mask or otherwise alter the patient's sensation of pain. For example, in many cases, patients report a tingling or paresthesia that is perceived as more pleasant and/or less uncomfortable than the underlying pain sensation. As used herein, unless explicitly stated otherwise, the terms “pulses” and “signals” are used interchangeably to include any waveform shapes, whether continuous or discontinuous, including but not limited to sinusoidal or non-sinusoidal waves such as square waves, triangle waves, sawtooth waves, etc.
One problem associated with existing stimulation systems is that aspects of the systems and/or the interactions between the systems and the patient may change over time. For example, the impedance of a stimulation circuit (which includes implanted electrodes and the patient's tissue) can change as scar tissue forms at the implant site and/or if the lead moves within the patient, and/or if the lead becomes disconnected or undergoes other changes. As the circuit impedance changes, the strength of the applied signal changes, which can reduce the efficacy of the signal and/or create patient discomfort. Accordingly, there remains a need for improved techniques and systems for addressing patient pain in a manner that is effective and comfortable over an extended period of time.